This invention relates to an optically coded keyboard arrangement for typewriters or similar office machines. To each key there is coupled a displaceably supported mask which can be shifted in a closed-curve path in the direction of the motion of the key and perpendicularly thereto. Each mask is actuated independently of the actuating speed of the associated key for performing a switching motion during which the mask, for a short period, interrupts or allows to pass through light beams between respective light sources and light detectors in accordance with the coded configuration of the mask. A separate switching spring is connected with each mask and is adapted to be bent through by the respective key upon actuation of the key.
In United States Patent Application Ser. No. 847,738, filed November 2nd, 1977 by Fred Johannsen, now U.S. Pat. No. 4,159,183, issued June 26th, 1979, there is disclosed an optically coded keyboard arrangement wherein on the machine frame there are arranged guide means with oblique camming faces which determine a closed geometrical path for the respective mask. Upon depressing and releasing a key, a guide pin secured to the respective mask glides along the oblique camming faces. The leaf spring-like switching spring which is, at one end, affixed to the key and is, in its middle, connected with the mask, is slidable at its free end in a guide fork. Upon motion of the key, the switching spring is tensioned by the guide means in opposite directions.